Living Mulch

Mulching garden beds has been a consistent ‘must-do’ for many gardeners.  Mulching reduces moisture evaporation, suppresses weeds, adds nutrients to the soil, reduces erosion and in many cases improves moisture absorption. Wood and bark chips are the most common materials employed.  Recently I have seen and read more about the negative effects of wood chip mulches. In particular when these mulches are over applied and allowed to compact, they can actually retard rain water absorption.  Thickly applied mulch can retard the growth and expansion perennials.   In his book The Know Maintenance Perennial Garden, Roy Diblik discusses how unnatural wood chip mulch is.  In areas that get adequate rainfall to support a tight matrix of plants, a living, or green, mulch is the ideal.

For many sustainable and naturalistic gardens the goal is to create this ‘green mulch’ or ‘living mulch’ by filling the space between garden features or larger plants with more plants.  ‘Green mulch’ is living plant material that performs all the functions of wood mulch such as weed suppression and soil moisture and temperature moderation.  In addition properly selected green mulches offer additional benefits like supporting wildlife and insects with food and cover, lower maintenance (since they do not to be replenished or broken up on a regular basis).  This green mulch may be a single species or a variety of plants that form an interlocking matrix. This matrix is most like what you would encounter in a meadow or woodland setting.

An example of a classically mulched bed. 
Each plant is distinctly identifiable and  there are no
random plants to confuse the composition.

One problem with green mulches is that they can look weedy due to lack of readability, no clear design or cluttered appearance.  Many folks are more comfortable with a landscape or garden where the features are clear and recognizable.  Having beds with clean edges is a quick way to make a garden more legible.  This human desire for legibility is also seen in the preference of many for plants neatly separated by oceans of mulch. 

One means of creating a green mulch is to install plants closer together so that they quickly grow together to create a continuous green carpet of foliage. This is one of the themes of the book New Naturalism, by Kelly Norris, published in 2021.  The trick here is to create plant communities that a good match to your site conditions and that the plants play well together. 

An example of a green mulch.  Creeping phlox, Phlox subulata,
and wild strawberry, Fragaria virginiana, have grown together
 to form a 4-6" deep ground cover.  Shrubs and perennials,
like the wild geranium, Geranium maculatum, here,
 are able to grow through.

Another route to a green mulch is to plant lower grow species as a matrix between your feature plants.  The trick here is to keep your garden readable so that the matrix does not obscure your design intent.  By way of example consider what bindweed does to a garden.  While it grows quickly to fill all the voids in the garden it also grows up and over taller plants resulting in tangle of plant material that has no discernible form, no starting or ending points and little textural contrast. (Kudzu does this on a much larger scale, draping a woodland edge with green vines, effectively removing all contrast between the trees.)  Consider instead a perennial bed where the spaces between plants are filled with low growing violets.  The glossy round leaves of the violets don’t interfere with the forms and textures of taller perennials or shrubs.  A list of some of the lowest growing  Mid-Atlantic natives includes:  Pussytoes (Antennaria sp.), wild ginger (Asarum canadense), Green and Gold (Chrysogonum virginianum), wild strawberry (Fragaria virginiana), barren strawberry (Geum fragarioides), Meehan’s mint (Meehania cordata), partridgeberry (Mitchella repens), golden ragwort (Packera aurea), various species of phlox (Phlox divaracata, stolonifera or subulata),  foamflower (Tiarella cordifolia), common violet (Viola soriana),  and  short sedges like rosy and ivory sedge (Carex rosea  and  C. eburnea ).

Golden ragwort is excellent native ground cover.  It spreads rapidly
 in moist, partly sunny locations.  It also seems to suppress invasive
 weeds like Japanese stilt grass and garlic mustard.

 

Using spring ephemerals for this purpose is a very natural means of creating a matrix.  These perennials naturally flourish early in the year when tree and shrub canopies are open, then slowly go dormant as the canopy closes up.  They return again the following spring.  In the Mid-Atlantic you may find Dutchman’s breeches and squirrel corn (Dicentra sp.), mayapple (Podophyllum peltatum)  and Virginia bluebells (Mertensia virginiana)  as common examples of these.  A limitation with using ephemerals is that they do totally disappear after setting seed.  They are ideal in an established perennial or shrub bed where you just need something in early spring while the larger plants are leafing out.  

Ephemerals, like cutleaf toothwort, Cardamine concatenata (in bloom),
 and Dutchman's breeches, Dicentra cucullaria, fill out in early spring.
but disappear as the tree canopy fills in. 

Using short-lived species that survive primarily by reseeding is another means to establishing a sustainable living mulch.  By nature these are opportunistic gap fillers.  As the longer lived perennial and shrub layers get established these reseeders tend to be squeezed out as their preferred, open habitat disappears.  Yellow fumewort (Corydalis flavula), American pennyroyal (Hedeoma pulegiodes), common yellow woodsorrel (Oxalis stricta), and self-heal ( Prunella vulgaris) are some of the shorter species that can be used in this way.  Taller, showier species that can be used as temporary gap fillers include wild columbine (Aquilegia canadensis), Partridge pea  (Chamaecrista fasciculata), fleabanes (Erigeron annuus and E. philadelphicus), and black-eyed Susans (Rudbeckia hirta).

Wild columbine, growing about 2' high, is relatively
 short lived in gardens due to competition and rich soils.
It produces a lot of seed so that it can reappear in other suitable
 locations where seeds can make soil contact and
have access to sunlight. (unmulched areas).

Yellow wood sorrel is considered a weed in many cases.  However,
 this native plant satisfies the requirements of a living mulch quite well. 
It grows quickly and produces lots of seed to fill gaps, is relatively short lived,
and is easily displaced by larger plants. As a native it also supports wildlife,
 particularly bees and birds.  Its lax, floppy habit detracts from it appearance,
making it appropriate around much larger plants and shrubs
or where this flop is not a distraction.

Oxalis stricta is very similar to its European relative, O. europea. 
Each has the same common name, common yellow woodsorrel. 
The North American species can be distinguished by the horizontal
disposition of the seed stalks (indicated by arrow). 
O. europea has seed pods on ascending stalks.

So if you are willing to give up the repetitive chore of mulching your garden beds, consider having your plants do that job for you and establish a living mulch.

Planting Plans for spring 2015

Every year brings another opportunity to grow my native plant collection.  This year I'm focusing on 4 areas:  Clean up and replanting around the swimming pool, clean up and expansion of the meadow, replacing the vinca along the driveway and build up a privacy hedge with the neighbor.  After learning a bit about the current conditions, what is already here and what might be expected to grow here naturally, I've put together a shopping list of natives to get this year.

This moss phlox is pretty happy growing along the pool deck,
A good portion of it is on the concrete slab.
The colors can be intense, so I use mostly one color at a time.

The area around the swimming pool is infested with common evening primrose (Oenothera biennis).  While native, there is just too much of it and it is not that attractive close up.  The soil is mostly a fast draining fill with a moderately high pH (ca. 7.5).  Since this is far from native soil I put a greater focus on what would look good growing in this setting.  Since I had already started using this area for plants native to Texas (my wife's home state) I will be adding two of my favorites, Wine Cups (Callirhoe involucrata) and Indian Blanket (Gaillardia pulchella).   I got some seed on a recent visit to the Wildflower Center.  These will be great for the full sun areas and they tolerate alkaline soils.  The moss phlox could use some bolstering up as well.  I am attempting to remove the English ivy from the enclosure and this phlox seems to be a good candidate to fill back in.

This species of wine cups grows close to the ground, filling gaps around taller plants.
Seed requires a hot water treatment and  30  days cold stratification for germination. 

This is the annual species of Indian blanket, Gaillardia pulchella.

Small's Penstemon is long blooming in shady locations.
The contrasting lavender and white blossoms show up at a distance.

In the shadier areas I will be trying out Greek Valerain (Polemonium reptans aka Jacob's Ladder) and Small's Penstemon (Penstemon smallii).  Despite its common name the valerain is actually a native to the of the US.  This had confused me for a while.  Looks like the name Greek valerain is used for a number of species in the Polemonium genus.  One of them with particularly showy flowers, P. caeruleum, is a European native.  P. reptans grows more like a ground cover

I've ordered some more Indian Pink (Spigelia marilandica) for an area in the pool enclosure with more moisture and organic soils.

This cottonwood seedling appeared in the vegetable garden.
I'll transplant it this spring to a moist part of the new meadow.

I have an area that has been overrun with invasives that I am trying to convert to a meadow.  I cleared half of it last year and hope to finish this spring.  After removing the bad guys I am backfilling with native species.  I realize that I'm making more work for myself by trying to kill off invasivies at the same time as introducing new plantings.  By planting mostly shrubs I think I can more easily manage the area with an annual mowing/whacking of the undesirable plants.  For the right way to convert a weedy area to a meadow or prairie check out this link.

Last year I planted an American plum (Prunus americana) and several elderberries (Sambucus canadensis).  This year I will add some chokecherries (Prunus virginiana) and smooth sumac (Rhus glabra).  I also have a Cottonwood (Populus deltoides) seedling to put on the edge.

The pawpaw blooms in early May,
just before the leaves open up.  

Way in the back I have a large grove of Pawpaw (Asimina triloba) trees.  I've been watching them for 2 years and I have yet to see any fruit.  Since pawpaws produce better with cross-pollination, I will be adding a couple of new individuals to the area,  It is possible that my entire grove is really just one clone.  We'll see if this helps, in a couple of years.

These goldenstar have more than doubled in size after a year in the ground.
I'll get some more to speed up coverage.

In the shady area around the driveway I have been ripping out the vinca and replacing it with shade tolerant natives.  I started with foamflower (Tiarella cordifolia) and Heucheras and these are taking hold.  In addition to these I've seen some really good results with Goldenstar (Chrysogonum virginianum, aka Green and Gold) in open shade areas.  These are relatively easy to find in a regular nursery, sometimes marketed with the plants that you can walk on.  Another plant that I've used in dry shade is big-leaf aster (Eurybia macrophylla), It has dense foliage and spreads by rhizomes so it should do a good job competing with the vinca.

This shiny summer foliage pf aromatic sumac turns red and orange in the fall.
We'll see how it performs in a shadier location.

The boundary between our nearest neighbor is defined with a double row of white pines.  At 40+ years old they are now limbed up fairly high and not providing much screening.  We have already put in a juneberry (Amelanchier canadensis), hazelnut (Corylus americana) and Hoptree (Ptelea trilfoliata).  There was also a pawpaw already there, doing a pretty good job despite the drier conditions.  We are looking to add some additional shrubs to fill in this gap and obscure the view.  A local native plant nursery has listed maple-leaf viburnum (Viburnum acerifolium) for sale this year.  I've been told that this is a very difficult shrub to propagate.  It does well in shady woods so I'm looking forward to trying it here.  I should get two since they don't self pollinate.  I am also looking to get some aromatic sumac (Rhus aromatica).  The wild form grows 5-12 feet, just the right size for our area.  I already have some of the 'Gro-low' cultivar.  At about 3 feet it is a great ground cover shrub for many difficult locations.

Now with may list in hand, I can hardly wait until spring!